1. Field of Disclosure
The present disclosure relates generally to regenerative heat exchangers and, more particularly, to rotary heat wheels for transferring sensible heat and water vapor between two counter-flowing air streams, when the warmer air stream is nearly saturated with water vapor and it is desired to heat and nearly saturate the cooler air stream. Even more particularly, the present disclosure relates to a desiccant-free heat and moisture exchange wheel, wherein wheel design and operating conditions produce large moisture transfers without employing desiccants conventionally used for such moisture transfers.
2. Description of Related Art
Regenerator heat exchange devices or regenerators are well known for effecting the transfer of heat and moisture between two counter-flowing air streams. One type of regenerator is the rotary air-to-air heat exchanger, which is typically in the form of a rotary heat exchange wheel including a matrix of heat exchange material. When rotated between counter-flowing air streams, the rotating wheel matrix is heated by the air stream with the higher temperature and, in turn, heats the lower temperature air stream. In addition, the rotating wheel may transfer moisture between the counter-flowing air streams. To promote moisture transfer, the wheel heat exchange matrix is usually made from, or coated with a moisture adsorbent desiccant material. Such heat exchange devices have been used in heating, ventilation and cooling (HVAC) systems for buildings, but have also been used for conditioning gaseous reactant streams for fuel cells.
In HVAC systems, rotary air-to-air heat exchangers are used to conserve energy within a building. During the heating season, such exchangers transfer heat and moisture from indoor air being exhausted to the outdoors to the cooler, dryer incoming fresh air. During the cooling season, such exchangers transfer heat and moisture from entering warm moist outdoor air to the cooler drier air as it is exhausted to the outdoors. Transfer of heat and moisture in this manner can typically reduce the amount of energy required to heat, cool, humidify or dehumidify the incoming ventilation air typically anywhere between about 50% and about 85%, depending primarily on the performance characteristics of the rotary heat exchange wheel employed.
Rotary heat exchange wheels have also been used to condition the gas flow circuit of a fuel cell system. Fuel cells generate electrical energy by chemical reaction. Examples of fuel cells include proton exchange membrane (PEM) fuel cells, phosphoric acid fuel cells, and alkaline fuel cells.
Fuel cells generally require two independent gas flow circuits for delivering reactant gases to an anode and a cathode of the fuel cell. The anode circuit feeds the fuel to the fuel cell, and the cathode circuit feeds the oxidant, typically ambient air, to the fuel cell. In order to maintain proper operating conditions for the fuel cell, the temperatures and humidities of the anode and cathode circuits must be carefully controlled to avoid drying out the electrolyte of the fuel cell, and thereby stopping the flow of electricity from the fuel cell.
U.S. Pat. No. 6,013,385 to DuBose, for example, shows a cathode humidification system including an enthalpy wheel, including a zeolite desiccant coating, for conditioning the oxidant. The enthalpy wheel operates by removing both sensible and latent heat from a cathode exhaust stream to heat and humidify a cathode inlet stream. DuBose states that the mass of the enthalpy wheel transports sensible heat, while the desiccant traps and transfers water vapor molecules and, thereby, latent heat. DuBose also shows varying the speed of rotation of the enthalpy wheel to vary the amount of moisture transferred to the cathode inlet, and using temperature, pressure, and relative humidity sensors to monitor the cathode inlet conditions and provide feedback control for the rotational speed of the enthalpy wheel.
As is known, an enthalpy wheel comprises a matrix of heat exchange material coated with a desiccant material capable of absorbing moisture in the form of water vapor. An enthalpy wheel is conventionally used where the transfer of both heat and moisture is desired. Suitable heat exchange materials are plastics (i.e., high molecular weight, synthetic polymers), aluminum, or papers made from either natural or synthetic fibers, while suitable desiccants are silica, alumina, and zeolites (molecular sieves). An enthalpy wheel may comprise a plastic strip coated with a desiccant material and wound in a spiral fashion around a hub.
A sensible wheel, in contrast, generally includes only a matrix of desiccant-free heat exchange media and is conventionally used where a transfer of heat alone is required. The prior art, including DuBose, has taught that an enthalpy wheel is required for transferring both heat and moisture efficiently between counter-flowing air streams, and that an enthalpy wheel should be used for such applications.
However, a sensible wheel has many practical advantages over an enthalpy wheel, if the sensible wheel can accomplish the required function. To begin with, a sensible wheel is generally easier and less expensive to manufacture, since a sensible wheel does not include a coating of desiccant over its sensible heat exchange media. In addition, many desiccants used in enthalpy wheels often adsorb and transfer contaminants found in the exhaust air stream along with the moisture. Furthermore, the desiccants themselves may detach in small pieces from the wheel and act as contaminants.
Accordingly, it would be desirable to have the option of using a desiccant-free heat exchange wheel in applications where both heat and a relatively large amount of moisture are to be efficiently transferred between counter-flowing air streams.
The present disclosure provides a system for conditioning a gaseous supply stream. The system includes a desiccant-free heat and moisture exchange wheel that efficiently transfers both heat and moisture between a warmer, substantially saturated gaseous exhaust stream and a gaseous supply stream such that the supply stream becomes heated and substantially saturated.
The presently disclosed conditioning system beneficially allows the use of a desiccant-free heat and moisture exchange wheel in applications requiring the transfer of both heat and relatively large amounts of moisture between counter-flowing air streams. The desiccant-free heat and moisture exchange wheel is designed in accordance with the present disclosure to transfer moisture through a process of condensation and re-evaporation. In general, a desiccant-free wheel provides the benefits of being easier and less expensive to manufacture than an enthalpy wheel, which is conventionally used in such heat and moisture applications, since an enthalpy wheel includes a coating of desiccant material over its sensible heat exchange media for trapping, transferring, and releasing non-condensing water vapor between counter-flowing air streams.
These and other features and benefits of the present disclosure will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures.